A DiskTreebank is a Collection of
Trees.
A DiskTreebank object stores merely the information to
get at a corpus of trees that is stored on disk. Access is usually
via apply()'ing a TreeVisitor to each Tree in the Treebank or by using
an iterator() to get an iteration over the Trees.

If the root Label of the Tree objects built by the TreeReader
implements HasIndex, then the filename and index of the tree in
a corpus will be inserted as they are read in.